Same here. I felt totally out of control. I had the "correct" springs, but no custom re-valving. I traded bikes with a friend that has a KTM 530 that has been tuned and with a steering stabilizer. He is about my same weight. I could not believe the difference. For the first time in my life I felt somewhat in control in the sand.
I then sent my suspension to Dale EO. He replaced the rear linear rate spring with a progressive, and re-valved for my weight and riding style. Now the front end is stable in the whoops and the sand. The rear end swims around in the sand, but I think that is due to my tires. I have DOT approved Dunlop 606, which are pretty stiff. When the rear tire wears out I am going to switch to a DOT approved trials tire.
I also added a steering damper. This is a first for me, so I may not have it dialed in properly yet. It wasn't obvious that it was helping in the sand, but I felt much less fatigued after riding rocky trails.
As other have stated, get the suspension dialed in first. A damper mask the stability problem, but proper suspension will fix it. Sag settings seem to be more important on the Bergs, so if you might play with that some.